1. Field of the Invention
The present invention relates to an illumination system, and more particularly an illumination system for wavelengths xe2x89xa6193 nm, such as that used for EUV lithography. The invention also relates to a method for reducing heat load, and a projection exposure apparatus comprising such an illumination system.
2. Description of the Prior Art
In order to be able to further reduce the line widths for electronic components, particularly in the submicron region, it is necessary to reduce the wavelengths of the light utilized for microlithograpy. With wavelengths smaller than 193 nm, for example, lithograpy with soft x-rays, so-called EUV lithography, is possible.
An illumination system suitable for EUV lithography should homogeneously, i.e., uniformly, illuminate a field used in EUV-lithography, particularly a ring field of an objective, with as few reflections as possible. Furthermore, a pupil of the objective will be illuminated independently of the field up to a specific filling degree "sgr", and an exit pupil of the illumination system will lie in the entrance pupil of the objective.
An illumination system for a lithography device, which uses EUV rays, has been made known from U.S. Pat. No. 5,339,346. For uniform illumination in a reticle plane and filling of a pupil, U.S. Pat. No. 5,339,346 proposes a condenser, which is constructed as a collector lens and comprises at least four mirror facets, which are arranged in pairs and symmetrically. A plasma light source is used as a light source.
In U.S. Pat. No. 5,737,137, an illumination system with a plasma light source comprising a condenser mirror is shown. In U.S. Pat. No. 5,737,137 an illumination of a mask or a reticle is achieved by means of spherical mirrors.
U.S. Pat. No. 5,361,292 shows an illumination system, in which a plasma light source is used. The plasma light source is point-like and is imaged into a ring-shaped illuminated surface by means of a condenser, which has five aspheric, eccentrically arranged mirrors. The ring-shaped illuminated surface is then imaged in an entrance pupil by means of a special sequence of grazing-incidence mirrors.
An illumination system is known from U.S. Pat. No. 5,581,605 in which a photon radiator is partitioned into a plurality of secondary light sources by means of a honeycomb condenser. A regular or uniform illumination is achieved in this way in a reticle plane. The imaging of a reticle on a wafer to be exposed is achieved with conventional reduction optics. Exactly one rastered mirror is arranged in the illumination beam path.
EP 0 939,341 shows a Kxc3x6hler illumination system for wavelengths  less than 200 nm, also particularly for the EUV range, with a first optical integrator comprising a plurality of first raster elements and a second optical integrator, comprising a plurality of second raster elements.
Another EUV illumination system has been made known from DE 199 03,807 A1. The system according to DE 199 03,807 A1 comprises two mirrors or lenses with raster elements. Such systems are known as double-faceted EUV illumination systems. The disclosure content of DE 199 03,807 A1 is incorporated to the full extent in the present application.
The construction principle of a double-faceted EUV illumination system is shown in DE 199 03,807 A1. An illumination in an exit pupil of the illumination system according to DE 199 03,807 is determined by an arrangement of the raster elements on the second mirror.
A disadvantage of the system known from DE 199 03,807 is that with field raster 5 elements, a light source with a diameter X is imaged directly into a pupil raster element. At the site of the pupil raster elements, in the ideal case, a stigmatic image of the light source is formed. With a small etendue of the light source, very small light-source images are formed on the pupil raster elements, so that a locally intense, concentrated, very high thermal load occurs therein.
Generally, the etendue or the phase space volume of the illumination optics is given by
(E) etendue=BXxc2x7BYxc2x7xcfx80xc2x72xc2x7NA2
wherein
BX: field width
BY: field length
NA: aperture of the imaging objective on the object side
: degree of coherence.
With typical values of BXxc2x7BY=8xc3x9788 mm2, NA=0.0625 and =0.8, an etendue of E=5.5 mm2 results. EUV light sources, such as a synchrotron light source, for example, have an etendue of approximately 0.001, thus smaller by a factor of 5500. In this respect, for example, reference is made to M. Antoni et al. xe2x80x9cIllumination optics design for EUV lithographyxe2x80x9d, Proc. SPIE, Vol. 4146, pp. 25-34 (2000). In such systems, the thermal load is distributed onto point-like light source images on a pupil raster element plate, whereby the overall power is reduced only slightly by the number of facet mirrors.
One object of the present invention is to provide a double-faceted illumination system, which is constructed as simply as possible and that has a reduced heat load on the second mirror or the second lens with raster elements, as well as a method for reducing the heat load on the second mirror with raster elements, sometimes referred to as xe2x80x9choneycombsxe2x80x9d, for such an illumination system.
The object is solved according to the invention for an illumination system for wavelengths xe2x89xa6193 nm, particularly for EUV lithography, in that the heat load is reduced by shifting the raster elements of the second mirror or of the second lens out of the focus of the light bundles of the raster elements of the first mirror or first lens with raster elements. Due to the divergence of the light bundles, a light bundle becomes broader with increasing distance from the focal point or from the image position of the secondary light source.
Since the diameter of a light bundle increases proportionally to the aperture angle of the light bundle, the following is valid:
xcex94D=2xcex94zxc2x7tan(xcex1)≈2xcex94zxc2x7sin(xcex1)≈2xcex94zxc2x7NA
with
xcex94D=change of the diameter of the light bundle
xcex94z=defocusing of the second raster elements
xcex1=half of the aperture angle of the light bundle
NA=numerical aperture of the light bundle
For a light bundle with diameter D(z=0)=X, the following is valid:
D(z)≈X+2xcex94zxc2x7NA
If one would like to obtain a predetermined filling degree of the second raster elements of the second mirror, the so-called pupil raster elements, then the amount of defocusing amounts to:             Δ      ⁢              xe2x80x83            ⁢      z        ≈                            filling          ⁢                      xe2x80x83                    ⁢                                    degree              target                        ·                          xe2x88x85              pupilhoneycomb                                      -        X                    2        ·        NA              =                              filling          ⁢                      xe2x80x83                    ⁢                      degree            target                          -                  filling          ⁢                      xe2x80x83                    ⁢                      degree                          z              =              0                                                  2        ·        NA              ⁢          xe2x88x85      pupilhoneycomb      
A defocusing xcex94z can be achieved by a change of the refractive power of the first raster elements or by a shifting of the second raster elements, whereby in the latter case, additional system data must also be fitted. Defocusing can occur to any extent, as long as the images of the light source, the so called secondary light sources, are not larger then the second raster elements.
In another advantageous embodiment, for achieving a particularly small local heat load, the amount of defocusing is determined in such a way that the extension of the secondary light sources is smaller than the size of the pupil raster elements, whereby the width of the non-illuminated edge region is less than 10% of the minimal diameter of the pupil raster elements. A non-illuminated region is a region in which the intensity is  less than 10% of the maximum intensity of the secondary light source.
By changing the points of incidence of the light channels passing through from the light source to the exit pupil, in a preferred embodiment, a specified illumination can be established in the exit pupil. Any desired distribution can be produced by such an adjustment of the light distribution in the exit pupil, and light losses, which occur, for example, when diaphragms are used for solving this problem, are avoided.
In systems according to the invention, with two optical elements with raster elements, the form of the pupil raster elements is adapted to the form of the secondary light sources and thus differs from the form of the first raster elements, the so called field honeycombs, also referred to as field raster elements. The pupil honeycombs, also referred to as pupil raster elements, are elliptical or round in a preferred embodiment, although the light source is configured as round.
In a preferred form of embodiment, additional optical elements, such as field mirrors, are arranged downstream to the mirrors with raster elements, and they serve for the purpose of imaging the pupil plane in the exit pupil of the illumination system, which coincides with the entrance pupil of the projection objective. Furthermore these elements form the ring field.
It is particularly preferred that the optical elements comprise grazing-incidence mirrors with angles of incidence xe2x89xa615xc2x0. In order to minimize the light losses associated with each reflection, it is advantageous if the number of field mirrors is kept small. Particularly preferred are forms of embodiment with at most three field mirrors.
Laser-plasma, plasma or pinch-plasma sources as well as other EUV light sources are conceivable as light sources for EUV radiation.
Other EUV light sources are, for example, synchrotron radiation sources. Synchrotron radiation is emitted, if relativistic electrons are deflected in a magnetic field. The synchrotron radiation is emitted tangentially to the electron path.